1. Field of the Invention
The present invention relates to a solid-state imaging device in which transfer electrodes and interconnections are made of polysilicon and a manufacturing method thereof, and to a solid-state imaging device in which a shunt interconnection layer is deposited on a transfer electrode and a manufacturing method thereof.
2. Description of the Related Art
Recently, solid-state imaging devices are widespread for use with audio-visual equipment such as a video camera or the like. In accordance with an increasing demand in which video cameras, for example, should become high in resolution and performance, solid-state imaging devices are requested to respond to an increasing demand for improvements of characteristics such as high-speed driving or the like.
In the solid-state imaging device, it is customary that transfer electrodes and parts such as interconnections connected to the transfer electrodes are made of polysilicon with application of semiconductor technologies.
However, since polysilicon is high in resistance value, it is unavoidable that a propagation delay occurs in polysilicon electrodes and polysilicon interconnections. As a result, solid-state imaging devices become difficult to operate at a high speed. Moreover, since the resistance value of polysilicon is high, polysilicon electrodes and polysilicon interconnections need sufficiently large cross-sectional areas. Accordingly, if areas of these polysilicon electrodes and polysilicon interconnections had been maintained sufficiently, it would not be easy to increase effective areas of the photosensor sections of the solid-state imaging device.
In order to obviate the above-mentioned defects, there have recently been proposed solid-state imaging devices including shunt interconnections as shown in FIGS. 1 and 2.
FIG. 1 shows a structure of a solid-state imaging device according to the related art in which a shunt interconnection 1 and a light-shielding film 2 are made of Al (aluminum). FIG. 2 shows a structure of a solid-state imaging device according to the related art in which a shunt interconnection 3 and a light-shielding film 4 are made of W (tungsten).
As shown in FIGS. 1 and 2, the solid-state imaging device includes a silicon substrate 5. On the surface-layer portion of this silicon substrate 5, there is formed a photosensor section (not shown) which converts a light signal into an electrical signal. As shown in FIGS. 1 and 2, the solid-state imaging device further includes a first transfer electrode 7 made of polysilicon and a second transfer electrode 8 made of polysilicon. The first and second transfer electrodes 7 and 8 partly overlap in the up and down direction with each other through an interlayer insulator film (not shown). The first and second transfer electrodes 7 and 8 have a buffer layer 9 made of polysilicon deposited thereon through an interlayer insulator film (not shown). The above-mentioned shunt interconnection 1 (3) is formed on the buffer layer 9 through an interlayer insulator film 10. The shunt interconnection 1 (3) is formed above the first and second transfer electrodes 7 and 8 in parallel to the first and second transfer electrodes 7 and B. The shunt interconnection 1 (3) is electrically interconnected to the first and second transfer electrodes 7 and 8 through a contact-hole or window (not shown) bored through the interlayer insulator film 10. Further, the shunt interconnection 1 (3) has an interlayer insulator film 11 deposited thereon so as to cover the shunt interconnection 1 (3). This interlayer insulator 11 has the light-shielding film 2 (4) deposited thereon such that the light-shielding film 2 (4) covers the first and second transfer electrodes 7 and 8.
In the solid-state imaging devices having the above-mentioned structures, since the shunt interconnection 1 (3) is is formed in parallel to the first and second transfer electrodes 7 and 8 made of polysilicon, their driving characteristics may be improved, and hence the solid-state imaging devices become able to operate at a high speed as compared with a solid-state imaging device in which the shunt electrode 1 (3) is not formed.
However, since the solid-state imaging devices shown in FIGS. 1 and 2 include the shunt interconnection 1 (3) and the buffer layer 9, the total thickness of the layers above the first and second transfer electrodes 7, 8 increases. In particular, in the solid-state imaging device shown in FIG. 1, the film thickness of the shunt interconnection 1 and the light-shielding film 2 made of Al (aluminum) increases comparatively, thereby resulting in shading of sensitivity being increased.
Further, since the number of the films comprising the transfer electrodes and the total thickness of the films comprising the transfer electrodes increase as described above, the photosensor section is damaged from a process standpoint when the respective films are removed on the portions above the photo-sensor section so that the resultant solid-state imaging device is damaged by increased illuminance defects.
Further, the transfer electrode of the solid-state imaging device is generally made of polycrystalline silicon. At that time, since the polycrystalline silicon of the transfer electrode has a high resist ce value and a resistance becomes maximum, in particular, at the central portion of a picture screen, there occurs a propagation delay to make it difficult to drive the solid-state imaging device at a high speed.
Furthermore, when it is intended to increase the area of the solid-state imaging device, since it takes plenty of time to transfer an electric charge from the pixel located at the end of the solid-state imaging device, it is also difficult to increase the area of the solid-state imaging device.
In order to solve the above-mentioned problem, there is proposed a solid-state imaging device in which an interconnection layer made of a metal having a low resistance value, i.e. a so-called shunt interconnection layer is formed on a transfer electrode through an insulating film and this shunt interconnection layer is connected through a contact portion to the transfer electrode. According to the above-mentioned arrangement, since a logic signal travels through the shunt interconnection layer having a low resistance value, the solid-state imaging device becomes able to operate at a high speed.
FIG. 3 is a cross-sectional view illustrating a portion near a light-receiving section of a CCD (charge-coupled device) solid-state imaging device 50 as an example of a solid-state imaging device in which a shunt interconnection layer is formed.
As shown in FIG. 3, this CCD solid-state imaging device 50 includes a semiconductor substrate 51 on which a light-receiving section formed of a photodiode or the like, a vertical charge transfer section for transferring an electric charge, a read section for reading a signal charge between the light-receiving section and the vertical charge transfer section and a channel-stop region for separating adjacent pixels although not shown. A transfer electrode 53 is formed on the region except the light-receiving section through an insulating film 54. FIG. 3 shows the cross-section of the portion in which the two transfer electrodes 53 overlap with each other through the insulating film 54.
Then, a shunt interconnection layer 55 made of a metal such as aluminum or the like is disposed on the transfer electrode 53 through the insulating film 54.
A light-shielding layer 56 is formed on the shunt interconnection layer 55 through the insulating film 54 so as to cover the whole of the imaging device. An opening 52 is defined on the light-receiving section through this light-shielding layer 56 and the light-shielding layer 56 is formed so as to cover the imaging areas other than the opening 52. In this CCD solid-state imaging device 50, the shunt interconnection layer 55 is connected to the transfer electrode 53, whereby the resistance value of the transfer electrode 53 may be decreased and the propagation speed may be increased.
However, in the above-mentioned arrangement in which the shunt interconnection layer is connected to the transfer electrode, a channel potential is changed due to the change of a work function of the transfer electrode, and a contact resistance at the contact portion between the transfer electrode and the shunt interconnection layer increases.